Eph receptor tyrosine kinases (Ephs) belong to a large group of receptor tyrosine kinases (RTKs), kinases that phosphorylate proteins on tyrosine residues. Ephs and their membrane bound ephrin ligands (ephrins) control cell positioning and tissue organization (Poliakov, et al., Dev Cell 7:465-80, 2004). In contrast to other receptor tyrosine kinases, Eph receptor activation does not only require ligand binding and dimerization, but also involves preformed ligand oligomers. Thus, tyrosine phosphorylation of Eph receptors requires presentation of ephrin ligands in their clustered or membrane-attached forms (Davis et al., Science 266:816-819, 1994). Functional and biochemical Eph responses occur at higher ligand oligomerization states (Stein et al., Genes Dev 12:667-678, 1998).
Among other patterning functions, various Ephs and ephrins have been shown to play a role in vascular development. Knockout of EphB4 and ephrin-B2 results in a lack of the ability to remodel capillary beds into blood vessels (Poliakov, et al., supra) and embryonic lethality. Persistent expression of some Eph receptors and ephrins has also been observed in newly-formed, adult micro-vessels (Brantley-Sieders, et al., Curr Pharm Des 10:3431-42, 2004; Adams, J Anat 202:105-12, 2003).
The de-regulated re-emergence of some ephrins and their receptors in adults also has been observed to contribute to tumor invasion, metastasis and neo-angiogenesis (Nakamoto, et al., Microsc Res Tech 59:58-67, 2002; Brantley-Sieders, et al., surpa). Furthermore, some Eph family members have been found to be over-expressed on tumor cells from a variety of human tumors (Brantley-Sieders, D. et al., supra); Marme, Ann Hematol 81 Suppl 2:S66, 2002; Booth, et al., Nat Med 8:1360-1, 2002).
Dominant-negative, soluble EphA2 or A3 proteins exhibit effects on ephrin-induced endothelial cell functions in vitro, and tumor angiogenesis and progression in vivo (Brantley, et al. Oncogene 21:7011-26, 2002; Cheng, et al. Neoplasia 5:445-56, 2003; Dobrzanski, et al. Cancer Res 64:910-9, 2004). However, because of lack of specificity of ephrin-A family members for Eph A receptors, these studies do not indicate whether EphA3 itself plays a role in the vascular endothelium in either tumor or normal tissues.
In summary, prior to the current invention, there has been no evidence that EphA3 is expressed on endothelial cells present in the tumor vasculature. Indeed, no vasculature abnormalities have been reported in EphA3 knockout mice (see, e.g., Vaidya et al. Mol. Cell. Biol. 23:8092-8098, 2003). Thus, although certain Eph receptors and ephrins have been implicated as playing a role in angiogenesis and tumor formation and progression, there have been no specific therapies that target EphA3 expression on tumor endothelial cells. This invention therefore provides new therapeutic targets and methods of treating tumors.